Add gitlab statistics similar to github

[imp]

If crate's git repo is hosted on github docs.rs shows projects stats fetched from github.com. This ticket is for having similar arrangement for gitlab.com as well

[severen]

The same would be nice for BitBucket as well (if it doesn't already exist), seeing as it another large hosting provider.

[Kixiron]

Triage: This is fairly low priority for us to do but if a PR were made we would accept it

[GuillaumeGomez]

I started to work on gitlab integration.

[jyn514]

Implemented in #1249.